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keyof and dynamic properties #14359

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@marzelin

Description

@marzelin

keyof works for dynamic properties where keys are strings (although not quite as expected) but not so much with numbers.

TypeScript Version: 2.2.1

Code

const getProperty = <T, P extends keyof T>(key: P, obj: T) => obj[key] 

interface Iobj {
  [key: number]: any
}

const obj: Iobj = {5: 5}
getProperty(5, obj) // Error: 'Argument of type '5' is not assignable to parameter of type 'never'.'

Expected behavior:

No errors (ts should infer that key is of type number here)

Actual behavior:

Error: 'Argument of type '5' is not assignable to parameter of type 'never'.'

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